This application corresponds to Japanese Patent Application Nos. 8-330826 and 9-275813, filed on Dec. 11, 1996 and Oct. 8, 1997, respectively, both of which are hereby incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates generally to overcurrent protection architecture and, more particularly, to thick-film resistor devices in communication networks and overcurrent protection circuitry employing the same. The invention also relates to overcurrent protection thick-film resistor devices and overcurrent protection circuitry employing the same, which devices are installed at interface sections between external cables of remote stations for implementing communication equipment and interstation communication equipment for use in protecting the station communication equipment against excess or overcurrent occurring due to application of surge voltages resulting from electrical storms (e.g., lightning) and/or accidental electrical contacts formed between external cables and power lines.
2. Description of the Prior Art
FIG. 5 shows one typical configuration of a prior known overcurrent protection circuit. In FIG. 5, the overcurrent protection circuit 10 is provided between two external cables 17, 18 and associated equipment to be protected (referred to as "to-be-protected" communication equipment) 19. The overcurrent protection circuit 10 comprises overcurrent protection thick-film resistor devices 11, 14, positive characteristic thermistors 12, 15 and variable resistors or "varistors" 13, 16. The overcurrent protection thick-film resistor devices 11, 14 and positive thermistors 12, 15 are respectively connected in series between external cables 17, 18 and the to-be-protected equipment 19. Interconnection nodes between the positive thermistors 12, 15 and the to-be-protected equipment 19 are coupled to ground via varistors 13, 16 respectively.
The configuration of a prior art overcurrent protection thick-film resistor device 11 is shown in FIG. 6. In FIG. 6, the overcurrent protection thick-film resistor device 11 includes an insulating or dielectric substrate 1 having a surface on which a thick-film resistor 2 is formed having a meander shape. An input terminal 3 and output terminal 4 are provided at ends of the thick-film resistor 2. Note here that since the overcurrent protection thick-film resistor device 14 has the same configuration as the overcurrent protection thick-film resistor device 11, an explanation thereof will be omitted herein.
The operation of the overcurrent protection thick-film resistor device 10 will now be explained with reference to FIGS. 5 and 6. In the overcurrent protection thick-film resistor device 10 of FIG. 5, upon application of a surge voltage (in which the potential is approximately 1 kV and the duration of the surge is less than or equal to about 1 ms) from the communication cables 17, 18 due to lightning or the like, resultant current can flow through the overcurrent protection thick-film resistor devices 11, 14 and positive thermistors 12, 15 and then to ground through varistors 13, 16, thereby ensuring that any surge voltage is not applied to the to-be-protected equipment 19. Since the overcurrent protection thick-film resistor devices 11, 14 are formed having an elongated meander-like shape, any disconnection or open-circuiting due to sparks will no longer take place. More specifically, by using an elongated meander-like resistor, a voltage applied to each unit length of the elongated resistor can be reduced. For instance, the voltage can be about 1/5 of a voltage applied to each unit length of a conventional square resistor. For this reason, as the length of the meander-like resistor is increased, the resistor provides better characteristics toward surge.
On the other hand, when the external cables 17, 18 are bought into contact (short) with an associated power line (e.g., an AC 600 V line or the like), excessive or overcurrent can be supplied from external cables 17, 18 to overcurrent protection circuit 10. In response to such overcurrent, the positive thermistors 12, 15 of overcurrent protection circuit 10 generate heat, increasing their electrical resistance, which thereby interrupts or cuts off the flow of such overcurrent. However, it may require some time before the positive thermistors 12, 15 reach their high resistance state caused by the generation of heat. This time lag may result in a risk that the overcurrent can flow into the to-be-protected equipment during such period. To mitigate this problem, the prior art circuit 10 is configured such that sparks occur at several portions of the meander-like thick-film resistor 2 in overcurrent protection thick-film resistor devices 11, 14, leading to a disconnection or open-circuit, thus eliminating application of overcurrent to the to-be-protected equipment.
However, in the above-described prior art, the exact location at which the sparks occur upon application of overcurrent to the overcurrent protection thick-film resistor device can not be predicted. Spark discharge or arc due to sparking of the overcurrent protection thick-film resistor device creates a high risk of negatively affecting associated parts or components disposed near or around the overcurrent protection thick-film resistor device. For example, these sparks can lead to electrical shorting, lead disconnection or the like in such associated parts or components. To prevent these problems, it is necessary that a constant distance be maintained between the site of the spark and such other components. Alternatively, certain techniques can be employed for forcing the interruption of spark discharge. Unfortunately, even in such cases, since the exact spark locations can not be specified or predetermined in advance, it is required that the other components be separated by a prescribed distance from the entire structure of the overcurrent protection thick-film resistor device. Alternatively, it is required that a casing or the like be provided as a coating means for completely coating or "wrapping" the meander-like resistor (e.g., the coating entirely envelopes the thick-film resistor device). However, this coating increases the size of the thick-film resistor device component, which reduces the spatial efficiency of the device. This coating may also increase the cost of this device.